@Alistair , I'm going to disagree with your statement,
"ROI on a back up generator = 0, so not that hard to beat."
Having a backup generator does have a pay out. I've lived through extended power outages, Hurricane Katrina to name one, where all the contents of refrig and freezer were lost. A generator keeps the cold stuff cold. Without power for 11 days, the house was not very livable without AC. Go to a hotel, if you can find one, most likely several hundred miles away.
With a generator, staying at your house helps to keep the potential looters at bay too. You simply can't put a value on the chance opportunity to shoot a looter. Priceless!